1. Field of the Invention
This invention relates to variable focus liquid-filled lenses and more specifically to polyphenyl ethers (PPE) that exhibit a high refractive index and low evaporation.
2. Description of the Related Art
Variable focal length lenses and microlens arrays are essential components for the next generation of consumer products such as mobile phones, security cameras, instrumentation, diagnostic equipment, imaging and optical communications. Dynamic tunability over a wide field-of-view (FOV) with minimal optical distortion is desired. One approach uses a liquid crystal cell to provide variable focal length when an external electric field is applied. These LC lenses are limited to small sizes, exhibit high optical distortion, slow response times and can provide a relatively small range of tunability. Another approach known as “electrowetting” applies a voltage across a drop of conductive fluid, which serves as a lens, to change the curvature of the drop, hence the focal length of the lens. This approach requires high voltages, is limited to small lens sizes and is highly susceptible to evaporation of the fluid.
An alternative approach was proposed by Sugiura et al. “Variable-focus liquid-filled optical lens,” Appl. Opt. 32, 4181, (1993) and Zhang et al. “Fluidic adaptive lens with high focal length tunability,” Appl. Phys. Lett. 82 (19), 3171-3172 (2003). They fabricated centimeter-scale liquid-filled optical lenses. Sugiura used an O-ring spacer to deform the elastic film and control lens focus. Zhang uses a syringe pump to inject fluid into the lens to control focus. The performance of these lenses is strongly influenced by the weight of the liquid, which induces non-symmetrical deformation of the lens surface when the lens is placed on a non-horizontal plane.
Considerable efforts have been made to reduce the size of the pneumatically activated liquid-filled lens to form microlenses and microlens arrays with micro-pumps using conventional lithographic processing. See N. Chronis et al. “Tunable liquid-filled microlens array integrated with microfluidic network” 22 Sep. 2003, Vol. 11, No. 19, Optics Express, pp. 2370-2378, J. Chen et al. “Variable-focusing microlens with microfluidic chip” J. Micromech. Microeng. 14 (2004) pp. 675-680, M. Agarwal et al. “Polymer-based variable focal length microlens system” J. Micromech. Microeng. 14 (2004) pp. 1665-1673 and I. Fernandez, “Variable Focus Microlens” Int. Pub. No. WO 2006/009514. In general, a chamber or array of chambers is formed over a microfluidic network, filled with a liquid and covered with a flexible membrane. A pneumatic pump applies pressure to the microfluidic network to inflate or deflate the membrane to control the focal length of each microlens. The most common membrane material is polydimethyl-siloxane (PDMS), which is a silicone-based elastomer.
The refractive power of the microlens for a given curvature is determined by the difference in the refractive indices of the fluid and air at the lens interface. The effect of the thin membrane is negligible and can be ignored.
For a thin, symmetrical, double-sided lens, the focal length, f, is given by 1/f=2/r*(nf−1) where nf is the index of the fluid and r is the radius of curvature of the surfaces. To maximize the tuning range of the focal length f for a given capability to vary the lens radius it is desirable to have a fluid with a high index of refraction. This will also reduce the amount of spherical aberration for a given focal length.
In general, the known liquid-filled lenses and microlenses use water or distilled water (n=1.33 at a wavelength of 0.63 microns) as their standard solution and mention that any high refractive index liquid could be used. Sugiura used a dimethyl silicon oil. Chronis suggests using microscope immersion oil (n=1.41) or UV curable polymer (n=1.56) as the optical liquid. Fernandez states that the fluid preferably comprises a polar liquid such as water or polyhydric alcohols and has a high surface tension.
Unfortunately, the selection of a suitable optical liquid is not as simple as just selecting any liquid with a high refractive index. For example oils typically have a high viscosity, which is harder to pump through the microfluidic network thus putting greater demands on the micro-pump and increasing the response time of the lens. Lenses that use UV curable polymers must be shielded from UV light to avoid curing the polymer. Polyhydric alcohols will evaporate through the membrane quickly.
In addition to having a high refractive index, suitable optical liquids must be non-toxic to humans, low viscosity to flow through the microfluidic network, low absorption into the membrane to avoid wrinkling and degrading the lens surface, low evaporation out of the membrane, low density for larger lens sizes and low dispersion at the wavelengths of interest. The Abbe number (v=(1−n589)/(n486−n656)), is a measure of dispersion in the visible band. The larger the Abbe number the less the dispersion. Water satisfies all of these criteria although it would be desirable to have a liquid with a higher refractive index and lower absorption/evaporation properties.
J. Lee et al. “Surface Modification of Poly(dimethylsiloxane) for Retarding Swelling in Organic Solvents”, Langmuir 2006, 22, 2090-2095 describes a method of alleviating swelling problems of PDMS molds in organic solvents. Lee attempts to solve or at least mitigate the problems of absorption of the liquid in the fluidic lens into the membrane and evaporation therefrom by modifying the PDMS membrane material.